An LED is a semiconductor based light source including a semiconductor diode and optionally photoluminescent phosphor material, also referred to herein as phosphor, for generating a light at a specified wavelength or a range of wavelengths. LEDs are traditionally used for indicator lamps and are increasingly used for displays, such as liquid-crystal displays (LCDs).
An LED emits light when a voltage is applied across a p-n junction formed by oppositely doped semiconductor compound layers. The wavelength of the light generated by the p-n junction depends on the band gaps of the semiconductor layers used to fabricating an active layer within the p-n junction of the LED. Thus, a specific p-n junction will emit only a narrow band of wavelengths. Additional phosphor materials are included in some LEDs as a coating over the LED. Light generated by the p-n junction that strikes the phosphors is converted up or down by the phosphors to a different wavelength. Thus, in addition to the wavelength of light emitted by the p-n junction, the LED emits other wavelengths from the phosphors. A typical white light LED, for example has a p-n junction that emits blue light. A portion of the blue light is converted to red and green light by the phosphors so that the total light output by the LED appears white.
As the LED is subjected to repeated use, the p-n junction within the LED begins to decay. As a result, over time the light intensity of the LED will drop. Further, the phosphors also decay at different rates with respect to each other and the p-n junction. Thus, the color of an LED with phosphors will also change with time.